Aqueous sea salt micro-aerosols play an important role in the heterogenous chemistry
of the lower marine troposphere both in polluted and in remote areas. In particular,
reactions with gases such as ozone or OH radicals leading to the release of molecular
chlorine have been intensely studied, both experimentally and theoretically.
Moreover,
thin layers of sea water deposited on Arctic ice
packs have been discovered to be a major
source of reactive bromine species which destroy the surface ozone layer
during polar sunrise.
There is increasing evidence
that the air-water interface is of a key importance in these chemical processes.
Despite this, little has been known about the structure and physical properties
of aqueous sea salt aerosols at a detailed, molecular level. Here, we summarize
results of
classical molecular dynamics, Car-Parrinello molecular dynamics and {\it ab initio}
quantum chemistry calculations on concentrated aqueous sodium chloride and bromide
solutions confined to cluster and slab geometries. The main questions addressed by
the simulations concern the onset of NaCl ionic solvation in water clusters, transition
from clusters to slabs, structure of solvation layers and degree of ion pairing
in concentrated solutions with confined geometries. A key result of the simulations
is the observation that polarizable halogen anions
(chloride and bromide) are present at the air-water
interface of bulk solutions in amounts sufficient for the
heterogenous atmospheric chemistry to take place.
The calculations also reveal that bromide actually exhibits surfactant activity,
i.e. its
concentration at the interface is higher than in the bulk. This is an accord with
the observed enhanced atmospheric reactivity of aqueous bromide compared to chloride
and with SEM experiments on wetting and re-drying of NaCl/NaBr co-crystals.